1. Field of the Invention
The present invention relates to an antenna apparatus, in particular, to an antenna apparatus on which a plurality of antennas are mounted and that can switch from one directivity direction to another for radio communication.
2. Description of the Related Art
In radio systems such as wireless local area network (LAN), for higher communication quality, a plurality of antennas are mounted and their directivity directions are switched. By optimally controlling the plurality of antennas, deterioration of transmission quality, for example, due to multipath fading can be suppressed. In addition, such systems can have resistance against disturbance from other radio devices and narrow the range in which the systems disturb other radio devices. For example, Patent Document 1, disclosed as Japanese Patent Application Laid-Open No. 2007-251677, describes a radio communication apparatus on which a plurality of antennas having different directivities are mounted.
FIG. 1 shows an exemplary structure of an antenna apparatus having a plurality of antennas as a related art reference. The antenna apparatus has a substrate 101 composed of a wireless module and so forth, a radio circuit section 102 disposed on the substrate 101, six antennas 103a, 103b, 103c, 103d, 103e, and 103f (hereinafter, they are generally designated by antennas 103 unless otherwise specified). This antenna apparatus is used in such a manner that the principal surfaces lay down.
As shown in FIG. 1, the six antennas 103 are disposed in such a manner that they form a circumference of a circle on a horizontal surface of one principal surface of the substrate board 101. Since the antennas 103 are nearly equiangularly disposed such that they divide the circumference of the circuit into six sectors, the antennas 103 can have directivities of different directions from each other.
In the radio communication using such an antenna apparatus, an optimum antenna 103 is selected from the plurality of antennas 103 corresponding to the exiting communication environment. Through the selected antenna 103, radio communication is performed. To reduce the influence of overlapping of adjacent antennas 103, it is necessary to isolate the selected antenna 103 from the adjacent antennas 103 for a predetermined amount. Thus, the spaces of the antennas 103 are secured corresponding to the communication frequencies of the radio system.
However, if the antennas 103 are disposed on the same plane as shown in FIG. 1, it is necessary to secure the spaces of the antennas 103 for their isolation. Thus, the overall structure of the antenna apparatus becomes large. This is a cause that prevents a product having the antenna apparatus from being downsized.
When this antenna apparatus is mounted on a rectangular-parallelepiped-shaped electronic device, if it is designed to lay down where the largest side of the six sides of the rectangular parallelepiped body is the bottom of the electronic device, it seems that the plurality of antennas 103 can be relatively easily mounted. In contrast, if the electronic device is designed to stand upright where the smallest side of the six sides of the rectangular parallelepiped body is the bottom of the electronic device, since the bottom area is not large, it becomes difficult to dispose the antennas 103. Thus, the shape of the product is restricted.
In addition, in the radio system that switches from one directivity to another of the plurality of antennas 103, as the number of antennas 103 is increased, the performance can be improved. However, if the number of antennas 103 is increased, the circumference of the circle in which the antennas 103 are disposed is increased. Thus, the space in which the antennas 103 are disposed is increased. As a result, since there is a tradeoff between the performance of the antennas and the area in which they are disposed, an antenna apparatus that allows the plurality of antennas 103 can be disposed in a saved space is being desired.
To solve such a problem, Patent Document 2, disclosed as Japanese Patent No. 3456507, describes a selector antenna that has a plurality of antenna elements disposed vertically, not horizontally and that allows the outer diameter (bottom area) of the sector antenna to be decreased without changing of directivity characteristics.